Former C-17 mechanic (part-timer)
It is a common thing. It happens when the thrust reversers are deployed. The blockers and cascades direct air up and forward, which upsets the flow of air entering the inlet and causes it to do this fun vacuuming of the flightline.
One of the coolest things I have ever seen was one annual tour graveyard shift standing ground for an engine run at 2 AM. The thrust reversers were deployed during the run, which I think was done so as not to blast the jet parked behind us, and a vortex formed at the number 3 engine. At the same time, the fan blades started to glow an eerie blue and St. Elmo's fire spread across the inlet. I said something into my headphones to the effect of "the engine is glowing blue" and the guy at the controls chopped power, disrupting the otherworldly scene. It was one of those moments that make me wish I had a camera hardwired into my optic nerve. Once everyone decided I had not been experimenting with recreational pharmaceuticals the run resumed, but the lightshow did not reappear.
Another fun fact: The C-17 can back up a 4% grade while fully loaded.